Range Of Incident Angles Research Articles

Sound propagation in the presence of non-isothermal sheared flow refraction and metasurface impedance reflection

Sheared flow can profoundly influence the aeroacoustic performance of near-wall structures like metasurfaces on moving vehicles. In this work, a refractive generalized Snell’s law is developed by combining Snell’s law with the generalized Snell’s law to manipulate anomalous reflections in an idealized sheared flow, comprising a uniform flow region and a stationary air region with velocity and temperature gradients. Theoretical analysis and numerical simulations of planar wave reflection by metasurface impedance under non-isothermal sheared flows are conducted, yielding favorable predictions of the zone of silence, as well as anomalous and total reflection behaviors. The total reflection between the shear layer and the metasurface is analyzed with the objective of confining incident wave below the shear layer. Remarkably, a specific configuration of hot-sheared flow and metasurface length can achieve total absorption over a range of incident wave angles from 0° to almost 90°. Moreover, the refractive generalized Snell’s law, extended to account for directional deflection induced by flow convection, is utilized to regulate sound beam reflection, resulting in satisfactory outcomes under sheared conditions. This study offers a theoretical framework for wave control under non-isothermal and sheared flow conditions, which cannot be resolved by the traditional generalized Snell’s law, and can be implemented in the design of metasurfaces for moving applications.

Reflectionless propagation of beams through a stratified medium

Reflectionless potentials following the prescription of Kay and Moses allow for total transmission of incoming waves of any kinetic energy. The optical analogue of such potentials occur as dielectric stratified media that can offer null reflectivity and near total transmission over a large range of incidence angles and wavelengths. In a previous work (Gupta and Agarwal, 2007), this was demonstrated for linearly polarized plane waves. We extend the earlier work valid for plane waves to structured beams to show near-total transmission of beams across the reflectionless dielectric profile. The analysis is based on the angular spectrum decomposition treating the beam as a collection of plane waves. Gaussian and Laguerre–Gaussian beams are shown to be transmitted through the film with <1% reflection in most scenarios. We also discuss the superlative performance of our proposed profile in preserving the beam shape during transmission comparing these results to a conventional λ/2 antireflection coating.

Quasi-BICs enhanced second harmonic generation from WSe2 monolayer

Abstract Quasi-bound states in the continuum (quasi-BICs) offer unique advantages in enhancing nonlinear optical processes and advancing the development of active optical devices. Here, the tunable robust quasi-BICs resonances are experimentally achieved through the engineering of multiple-hole Si-metasurface. Notably, the quasi-BICs mode exhibits flat bands with minimal dispersion at a wide range of incident angles, as demonstrated by the angle-resolved spectroscopy measurements. Furthermore, we demonstrate a giant second-harmonic generation (SHG) enhancement by coupling a WSe2 monolayer to the quasi-BICs hosted in the metasurface. Leveraging the strong local electric field and high state density of the observed quasi-BICs, the SHG from the WSe2 monolayer can be enhanced by more than two orders of magnitude. Our work paves the way for effectively enhancing nonlinear optical processes in two dimensional (2D) materials within the framework of silicon photonics and is expected to be applied in nonlinear optical devices.

Physical aspects of vortex-shock dynamics in delta wing configurations

Delta wing configurations with double- and triple-leading edges introduced within the North Atlantic Treaty Organization Applied Vehicle Technology -316 task group are examined to investigate the dynamics of vortices and shocks, with potential implications for the preliminary aircraft design. The numerical simulations are conducted for the configurations at Ma∞=0.85 and Re∞=12.53×106 using the Reynolds-averaged Navier–Stokes k−ω shear stress transport (SST) model across a range of incidence angles. The detailed analysis focuses on the case with α=20° using the scale-adaptive simulation based on the k−ω SST model. This study considers shock-vortex interaction and breakdown with buffeting to study the transient flow physics over the wing. Additionally, insights into vorticity strength and destruction are gained through the enstrophy transport equation. The findings reveal that the inboard vortex (IBV) development is impeded by counter-rotating secondary vortices from IBV and the midboard vortex. A key distinction is observed for the first time between the double-delta and triple-delta wings, in that the double-delta wing experiences shock-induced vortex breakdown, with the transient nature of this breakdown leading to an adjustment in the shock position, causing a shock buffet. In contrast, the breakdown in the triple-delta wing is linked to a stationary shock induced by the kink in the planform. This study highlights the crucial role of the orientation of the shock relative to the vortex axis in characterizing the aerodynamic performance of the planforms.

Silicon nitride-based ultra-wideband ultra-long infrared metamaterial absorber with large angle and high absorption

In the study of mid-infrared band long wavelength infrared (LWIR) has attracted the attention of domestic and foreign researchers due to its great role in medical diagnosis and detection. Broadband absorption is also a key index for metamaterial absorption applications. In this paper, a design of a highly absorbing, large-angle ultra-wideband ultra-long infrared metamaterial absorber based on a single-layer square Si3N4 cavity array and an intermediate layer of Si3N4 thin film, as well as metal Ti as the substrate, is proposed. The ultra-long infrared metamaterial absorber has polarization insensitivity under the condition of positive incidence. Numerical simulations show that the effective absorption bandwidth with more than 90% absorption is 11.12 μm in the band range of 7.65–18.77 μm, in which the average absorption reaches 96.54%. Diffraction and impedance matching induced strong absorption and wide bandwidth. We can adjust the absorption performance by changing the structural size of the absorber. In addition, our design has a very wide operating range of incidence angles, and the TM and TE modes maintain ultra-high average absorption levels under 0°-60° incidence angles, it is worth mentioning that the absorption of the absorber can reach 95.08% at an incident angle of 60°. The absorber design proposed in this paper has a simple structure, the material is easily available and inexpensive, and the fabrication process is very simple, which makes it suitable for infrared imaging, medical detection, and thermoselectronic devices.

Wide-band microwave absorber with polarization insensitivity and incident angular stability using resistive ink for stealth application

A wide-band microwave absorber using five layers of resistive ink patterns is designed with a periodicity of 0.15λL, where λL denotes the wavelength at the lowest operating frequency. Symmetrically shaped resonators are patterned on five layers of FR4 substrates with air spacers between them. The fractional bandwidth of the absorber is 140% with absorption greater than 90% attained in the operating frequency range from 3.38 GHz to 19.5 GHz. Cross-polarized absorption of the structure is found to be less than 0.1%. Absorption is stable for a wide range of incident angles up to 40∘. Absorption behavior is found to be stable for both TE and TM polarized incident waves under normal incidence. Further, the mono-static radar cross-section is analyzed and found to be reduced below -40 db in the entire operating frequency.

Planar Polarization Rotation Pattern Manipulation Surfaces With Enhanced Incident Angle Range and Beam-Steering Ability

Planar Polarization Rotation Pattern Manipulation Surfaces With Enhanced Incident Angle Range and Beam-Steering Ability

Crossed Andreev reflection in normal-superconductor-normal junction based on Kekulé-Y patterned graphene

We theoretically study the crossed Andreev reflection (CAR) of the normal metal-superconductor-normal metal (NSN) heterojunction based on Kekulé-Y patterned graphene with two doping types, i.e. nSn and nSp configurations. It is found that the enhanced CAR is more likely to occur in the nSp junction rather than the nSn junction. To be concrete, the almost perfect CAR occurs in a large range of incident angle in the single Dirac cone phase when the incident energy is inside the gap of the nonlinear band. Furthermore, the roles of the length of superconductor and pseudospin-valley coupling on conductance are also evaluated.

Data-driven modeling and fast adjustment for digital coded metasurfaces database: Application in adaptive electromagnetic energy harvesting

Metasurfaces (MSs) show great promise in efficient electromagnetic energy harvesting (EMEH) due to their compactness, high efficiency, and long-distance transmission capabilities. Nonetheless, the conventional iterative and time-consuming solving process of MSs significantly escalates computational demands. Furthermore, once processed, the MS shape remains fixed and cannot be adapted to changing requirements. Accordingly, a critical challenge is the development of a new efficient solver for MS real-time tuning. Here, we introduce a class of digital coded MS databases including multiple pre-defined resonant frequency MS. The combination of multiple MS base functions from the database enables swift resonance frequency adjustments to adapt to changing environmental conditions. A topology optimization method based on data-driven modeling is employed to rapidly acquire the optimal digital coding for the corresponding MS at various operating frequencies, facilitating the construction of a database. This approach integrates a convolutional neural network and genetic algorithm (CNNGA). It not only enables more accurate and expedited forward prediction of MSs' electromagnetic (EM) response but also facilitates inverse design based on specified requirements. We employ this method to design a MS that achieves perfect energy harvesting (EH) over a broad range of incident angles and polarization directions. In addition, a data-driven modeling is used to establish an EH efficiency predictive model corresponding to MS combination. This model serves as a guide for real-time MS adjustments as per changing requirements. Compared to previously designed MSs, this model achieves rapid design and adaptive adjustment capabilities. Through the incorporation of various functional MS base functions into the database, this method can be universally applied to MS combinations tailored to specific functions, including EM cloaking, ultra-thin flat lenses, and computational MSs.

Coupling optimization design of configuration and swept-bowed effects in highly loaded subsonic compressor tandem cascades with tip clearance

The coupling optimization design for swept-bowed effects and configuration of tandem cascade is conducted in a highly loaded subsonic compressor tandem cascade, in which the novelty lies in considering the spanwise distribution of configuration parameters and exploring the coupling relationship between them and swept-bowed design. Research results show that, compared with the original one, the loss coefficient of the optimal tandem cascade is decreased by 38.7% at the 4° incidence angle and the range of available incidence angle expands by 25%. For the distribution of the configuration parameters, parameters at the hub are significantly different from those near the tip, which confirms the tandem blade design in compressors is complex and needs to consider 3D flow effect. Forward swept and positive bowed design are beneficial for reducing loss at tip and alleviating the blockage, which cannot be achieved by the optimization only for configuration parameters. In terms of the weak coupling relationship between configuration parameters distribution and swept-bowed effects, the former should be considered first in practical engineering.

Wide-angle ultrabroadband absorption in black phosphorus/dielectric multilayer anisotropic metamaterials

An ultrabroadband far-infrared absorber is achieved using an anisotropic metamaterial composed of alternating black phosphorus (BP) and dielectric films arranged in a trapezoidal structure. We numerically demonstrate that ultrabroad bandwidths (with >90% absorptivity) can be achieved with the strong anisotropic dielectric response of BP, namely 63.6 μm along the armchair direction and 53.6 μm along the zigzag direction. Importantly, the high absorption is maintained across a wide range of incident angles. Our simulation results align well with analytical calculations based on the effective medium theory, considering the multilayer structure as an effective homogeneous metamaterial with anisotropic permittivity. From the distribution profiles of magnetic fields, we observe tight trapping of different wavelengths at varying widths of the trapezoidal absorber, revealing the slow-light effect underlying the broadband absorption. Our study holds significant potential for device applications, such as BP-based broadband infrared photodetectors.

Estimating structural response to bidirectional seismic excitation using spectrum compatible motions: Performance of a pair of companion spectra

AbstractBidirectional analyses covering a range of incidence angles for several records are essential yet challenging for routine design. Recent studies have shown that the response to bidirectional shaking may be estimated per straightforward unidirectional analysis in the most preferred orientation in conjunction with the orthogonal combination rules, and this can also remove the need for various incidence angles. Further, it is well known that the mean response can be estimated with reduced dispersion using fewer spectrum‐compatible records. Combining the advantages of both, a pair of companion spectra independent of period and orientation has recently been developed. While the basis of the companion spectra is established therein from a sound mathematical perspective, the applicability of the proposed spectra to real structures may be revisited, especially to structures sensitive to bidirectional interaction, before recommending the companion spectra for practical design. To achieve this end, following a brief scrutiny of the companion spectra, the performance of the same is critically examined for idealized SDoF analogues and real structures such as bridge piers representative of SDoF systems and buildings with asymmetry using single storey and multistorey models (MDoF systems). The results show that the response of the real structures to bidirectional seismic loading can be reasonably estimated with reduced dispersion using fewer records compatible with the pair of the companion spectra. As such, the critical response parameters may be computed by actual bidirectional analyses or by combining responses to unidirectional analyses under fewer spectrum compatible records. Hence, the pair of companion spectra that can be used as “target” to independently match two components of a chosen record is useful for practical design.

Simple numerical X-ray polarization models of reflecting axially symmetric structures around accreting compact objects

ABSTRACT We present a series of numerical models suitable for X-ray polarimetry of accreting systems. First, we provide a spectropolarimetric routine that integrates reflection from inner optically thick walls of a geometrical torus of arbitrary size viewed under general inclination. In the studied example, the equatorial torus is illuminated by a central isotropic source of X-ray power-law emission, representing a hot corona. Nearly neutral reprocessing inside the walls is precomputed by Monte Carlo code stokes that incorporates both line and continuum processes, including multiple scatterings and absorption. We created a new xspec model, called xsstokes, which in this version enables efficient X-ray polarimetric fitting of the torus parameters, observer’s inclination and primary emission properties, interpolating for arbitrary state of primary polarization. Comparison of the results to a Monte Carlo simulation allowing partial transparency shows that the no-transparency condition may induce different polarization by tens of per cent. Allowing partial transparency leads to lower/higher polarization fraction, if the resulting polarization orientation is perpendicular/parallel to the rotation axis. We provide another version of xsstokes that is suitable for approximating nearly neutral reflection from a distant optically thick disc of small geometrical thickness. It assumes local illumination averaged for a selected range of incident angles, representing a toy model of a diffuse corona of various physical extent. Assessing both xsstokes variants, we conclude that the resulting polarization can be tens of per cent and perpendicularly/parallelly oriented towards the rotation axis, if the reflecting medium is rather vertically/equatorially distributed with respect to a compact central source.

Investigation of control effects of end-wall self-adaptive jet on three-dimensional corner separation of a highly loaded compressor cascade

To overcome the limitations posed by three-dimensional corner separation, this paper proposes a novel flow control technology known as passive End-Wall (EW) self-adaptive jet. Two single EW slotted schemes (EWS1 and EWS2), alongside a combined (COM) scheme featuring double EW slots, were investigated. The results reveal that the EW slot, driven by pressure differentials between the pressure and suction sides, can generate an adaptive jet with escalating velocity as the operational load increases. This high-speed jet effectively re-excites the local low-energy fluid, thereby mitigating the corner separation. Notably, the EWS1 slot, positioned near the blade leading edge, exhibits relatively low jet velocities at negative incidence angles, causing jet separation and exacerbating the corner separation. Besides, the EWS2 slot is close to the blade trailing edge, resulting in massive low-energy fluid accumulating and separating before the slot outlet at positive incidence angles. In contrast, the COM scheme emerges as the most effective solution for comprehensive corner separation control. It can significantly reduce the total pressure loss and improve the static pressure coefficient for the ORI blade at 0°-4° incidence angles, while causing minimal negative impact on the aerodynamic performance at negative incidence angles. Therefore, the corner stall is delayed, and the available incidence angle range is broadened from −10°–−2° to −10°–4°.This holds substantial promise for advancing the aerodynamic performance, operational stability, and load capacity of future highly loaded compressors.

Dual-band flexible metamaterial absorber working in 5G band

This article presents a thin and flexible artificial electromagnetic absorber with two absorption peaks at frequencies of 28 and 39 GHz for 5G communication. Copper and a flexible FR4 substrate are used in a pyramid-shaped layered configuration, achieving absorption rates exceeding 99% at 28 and 39 GHz. The thickness of the proposed structure is only 0.34 mm, which is ∼1/32 and 1/23 of the corresponding free-space wavelengths at the absorption frequencies. Due to its symmetric structure, the absorber is polarization-insensitive. Furthermore, the proposed structure exhibits over 90% absorption within the incident angle range of −45° to 45°. To investigate its absorption mechanism, the electric field and magnetic field distributions at the absorption frequencies were analyzed. Then a 150 × 150 mm2 sample was fabricated using printed circuit board manufacturing techniques. This sample was placed on flat or curved surfaces and tested under normal or oblique incidences. The experimental results were consistent with the simulated results, confirming the feasibility of the design. This absorber can be applied in 5G communication and millimeter-wave imaging.

Minimizing annual reflection loss in fixed-tilt photovoltaic modules using graded refractive index (GRIN) anti-reflective glass

This study evaluates the performance of graded refractive index (GRIN) anti-reflective (AR) structures on photovoltaic (PV) modules across twenty global locations and compares them with conventional thin film AR coatings and bare glass. Optimized glass nanocones were chosen to represent the GRIN structures. Electrodynamic simulations were conducted to assess reflection properties at multiple angles of incidence. GRIN AR structures dramatically reduce total solar-integrated reflection to 0.2% at normal incidence, compared to 3.8% for bare glass and 0.8% for optimized thin film AR coatings, and maintained low reflection across a range of incident angles, demonstrating only 0.7% reflection at 60 degrees compared to significantly higher values for other surfaces. Detailed hourly simulations for twenty global locations revealed that GRIN AR coatings substantially reduce annual reflection loss to just 0.86±0.19% and increase expected annual energy output by up to 11.15±1.00% compared to bare glass at optimal tilt angles. The benefits of GRIN AR coatings are more substantial at sub-optimal panel orientations such as horizontal or vertical due to its broad angle AR properties. These findings highlight the potential of GRIN AR technology to approach the theoretical limit where front surface reflection loss is completely eliminated and enhance solar power conversion efficiencies. Additionally, a generalized model to approximate annual reflection loss for various materials is presented. This research encourages further exploration into GRIN AR coatings to optimize power conversion efficiencies and reduce reflection in PV modules.

Efficient and high-quality absorption enhancement using epsilon-near-zero cylindrical nano-shells constructed by graphene

This paper presents a detailed scattering analysis of a hollow-core plasmonic-shell cylindrical wire to design an efficient, compact, narrowband, and reconfigurable optical absorber. The shell is formed by a thin graphene material, investigated in its epsilon-near-zero (ENZ) plasmonic region. Compared to the graphene plasmonic resonances in the terahertz(THz)/far-infrared (FIR) frequencies, the ENZ plasmonic resonances offer a blue shift in the operating frequency of the second-order plasmonic resonances by increasing the geometrical dimensions. This feature is successfully used to design efficient optical wave absorbers with absorption cross-sections much larger than geometrical and scattering cross-sections. The observed blue shift in the resonance spectrum, which is the key point of the design, is further verified by defining each particle with its polarizability and fulfilling the resonant scattering condition in the framework of Mie’s theory. Furthermore, graphene relaxation time and chemical potential can be used to manipulate the absorption rate. Observed resonances have narrow widths, achieved with simple geometry. To consider more practical scenarios, the one-dimensional arrangement of the cylindrical elements as a dense and sparse array is also considered and the design key point regarding graphene quality is revealed. The quality factor of the sparse array resonance is 2272.8 and it demands high-quality graphene material in design. It is also observed that due to the use of small particles in the design, the near-field and cooperative effects are not visible in the absorption cross-section of the array and a clear single peak is attained. This polarization-insensitive absorber can tolerate a wide range of incident angles with an absorption rate above 90%.

Vanadium dioxide-based terahertz metasurface device with switchable broadband absorption and beam steering functions

A simple terahertz (THz) metasurface device with dual functionalities of broadband absorption and beam steering is proposed in this paper. The device exploits the phase change properties of vanadium dioxide (VO2), transitioning between its insulating state and metallic states to achieve functional switching. In the metallic state, the device exhibits broadband absorption, surpassing 90% absorption in the 1.03–2.07 THz range. The absorption function maintains exceptional performance across a wide range of incident angles and possesses polarization insensitive properties. In the insulating state of VO2, the metasurface converts into a THz reflective beam steering device. By employing phase coding, 1-bit coding metasurfaces based on phase gradients are designed, enabling dual-beam and four-beam reflection. Building upon this foundation, 2-bit coding metasurfaces facilitate anomalous reflection of a single beam in distinct directions. Consequently, the device not only toggles between broadband absorption and beam steering but also satisfies the design criteria for 3-bit coding metasurfaces. The modulation characteristics of this device serve as a valuable design reference for multifunctional THz devices in diverse scenarios.

Enhancing 5G propagation into vehicles and buildings using optically transparent and polarisation insensitive metasurfaces over wide-incidence angles

This article introduces two transmissive metasurfaces applied to normal windows, aiming to improve the 5G outdoor-to-indoor (O2I) coverage. These windows can be utilized in various settings, such as vehicles or buildings. The proposed unit cells, designed to be wide-incident angle and polarization insensitive, are implemented in both single-glazing and double-glazing glasses, arranged in a periodic structure to form the transmission surfaces. Both metasurfaces maintain optical transparency by incorporating Indium Tin Oxide (ITO) as the conductive element in each unit cell. These engineered transmission surfaces enhance the 5G signal indoor coverage at the 3.5 GHz band across a broad range of incident angles. While multi-layer structures typically exhibit heightened sensitivity to the angle of incidence, the proposed two-layered transmissive surfaces demonstrate substantial angular stability, reaching up to 65 and 75 degrees for double- and single-glazed glass, respectively. To achieve this wide and stable angular response, evolutionary optimization techniques were employed to fine-tune the proposed unit cells. Both designs exhibit a high transmission coefficient across the operating frequency for a variety of incident angles, surpassing those reported in the existing literature. Experimental evaluations of the fabricated prototypes indicate that both metasurfaces hold significant potential for enhancing signal propagation into buildings and vehicles.

Metasurfaces for sound absorption over a broad range of wave incidence angles

Control of reflected waves that meet surfaces from oblique directions is crucial, for instance, in closed spaces. Metasurfaces composed of Helmholtz resonators can be efficient and compact absorbers but have limited ability to achieve high absorption over a wide incidence angle range, especially when designed for high performance in the region approaching grazing incidence. In turn, sonic crystals can be used to manipulate wave propagation direction at low frequencies. We propose a type of absorber that combines a surface of 2D Helmholtz resonators and a 2D sonic crystal with cylindrical scatterers arranged in a hexagonal lattice. The combined effect of both structures yields a metasurface that can achieve high absorption over a broad range of incidence angles. Here, an analytic model to estimate the behavior of the absorbers for wavelengths that are much longer than the unit cell dimensions is presented. The model is used in combination with an optimization strategy to realize designs for single frequency and octave-band performance. The test cases show that surfaces with absorption coefficient values above 0.9 for the range of incidence angle extending from 0∘ until 83∘ can be realized. The performance of the absorbers is verified with a finite element model and experimentally.